The present invention relates to pulse width modulation and more specifically to filtering pulse width modulated waveforms.
In many cases it is not feasible to impose a DC or low-frequency waveform directly on a given system. Instead a pulse width modulated (PWM) waveform may be produced and then averaged using a low-pass filter.
It is known in the prior art to use a PWM waveform for producing a high-frequency waveform of a desired average. The analog representation can be recovered by demodulating/filtering the waveform. However, as the bandwidth content of the average waveform approaches the carrier frequency, the requirements of the recovery filter become stricter. The filter must reject the carrier frequency while passing through the desired signal. One means to design such a filter is to create a high order low pass linear time invariant filter. One drawback of such a design is phase lag. The higher the order of the filter, the more accurate the filter is, but the greater the phase lag that is introduced which is undesirable in a feedback control system.
Another approach to retrieving the analog version of the PWM waveform is to integrate the PWM waveform over one PWM period and to calculate the average value over one cycle. An embodiment of such a circuit is shown in FIG. 1 as taken from U.S. Pat. No. 5,619,114. Such a circuit requires an integrator, amplifier, and a calibration scheme to account for component tolerance. Thus, there is a need for a simpler system and method for obtaining the analog representation of a PWM waveform, when frequency content of this analog representation is significant when compared to the PWM frequency.